This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have found that: Estrogen and drugs that bind to the estrogen receptor interfere with inhibitory learning in female but not male rats. The growth factor brain derived neurotropic factor increases in different parts of the pathway that allows an odor to activate the brain's fear center called the amygdala. Different subunits of a neurotransmitter called glutamate affect learning of conditioning fear. The opiates morphine and buprenorphine block conditioned fear and both show tolerance to this effect. A peptide called Substance P blocks conditioned fear when infused locally into different parts of the neural pathway that connects the medial nucleus of the amygdale to the acoustic startle reflex pathway in the brainstem.